__traits(compiles) and template instantiation

[jmh530 via Digitalmars-d-learn]

I've been playing around with __traits and I find myself confused 
on one aspect. In the code below, I was testing whether some 
templates would compile given types. For the most part it works 
as I would expect.

I think I get why the third one works with foo!(int). My guess is 
that it assumed that U is the same as T and both are int. 
However, that wouldn't make sense with the last one where I use 
bar!(int). In that one it's basically ignoring the second 
template constraint. So I don't understand what's going on for 
that last line to compile. To confirm I wasn't crazy, I get an 
error with
alias bar_ = bar!(int);



import std.traits : isNumeric;
import std.range : isInputRange;

void foo(T, U)(T x, U y) if (isNumeric!T && isNumeric!U) { }

void bar(T, U)(T x, U y) if (isNumeric!T && isInputRange!U) { }

void main()
{
	assert(__traits(compiles, foo!(int, int)));	//I get this
	assert(!__traits(compiles, foo!(bool, bool)));	//I get this
	assert(__traits(compiles, foo!(int)));		//I think I get this
	assert(__traits(compiles, bar!(int, int[])));	//I get this
	assert(!__traits(compiles, bar!(int, int)));	//I get this
	assert(__traits(compiles, bar!(int)));		//I don't get this
}